Lean Engine Codes

[brothapig]

Well, for a while my car's been throwing some codes. Engine light will stay on for a day or two, then go out for a while. I finally took it to AutoZone to check it. I don't know the code numbers, but here are the descriptions:

1) Lean exhaust, bank 1
2) Lean exhaust, bank 2
3) Multiple cylinder misfire

I don't notice any misfires, but I *think* the code come on when I am decelerating. Some ideas that I have:

-fuel pump
-dirty injectors
-bad O2 sensors
-TPS (though I don't even know if the firebird has one )

I'm leaning away from anything that wouldn't affact the entire engine (i.e. injectors, O2 sensors...what's the chance they would go all at once?)

Any other ideas out there? Anyone else have similar codes?

The car otherwise runs fine; no hesitation, no missing (that I can tell), starts fine, etc.

Ryan


Just thought of something...could it be the fuel filter? And where are they on the F-bodies? Please don't tell me with the fuel pump in the tank...

[Injuneer]

Always difficult without the exact codes, but I'll guess you had:

P0171 - Bank 1 Fuel Trim System Lean
P0174 - Bank 2 Fuel Trim System Lean

That means the long term fuel corrections have max'd out and can't control what the O2 sensors/PCM see as a lean condition. This can be caused by low fuel pressure, faulty MAF sensor, large vacuum leaks, exhaust leaks, severe misfires, faulty O2 sensors.

A "scan" using data logging software like AutoTap would probably help to find the problem. Lacking that, start with the easiest which is to check the fuel pressure - should hold at least 40psi at WOT/max load. You might also be able to check the MAF sensor by unplugging it and letting the PCM run in speed-density mode. If the problem goes away, it would point to low readings out of the MAF sensor (e.g. - oil coating on the elements).

The misfire detection is based on the CKP sensor measuring the crankshaft speed, and the PCM comparing the irregularities in the velocity to the known pattern for the engine. Its very sensitive, and can pick up things you might not (it can also be fooled by large cams, bumpy road surfaces, loose reluctor, etc.). But it would take large misfires on both sides of the engine to cause the PCM to set the lean codes on both sides. And, the engine would be running extremely rich, to the point where the exhaust would smell of fuel.

[brothapig]

I stopped by Autozone again and wrote down the code numbers this time. Injuneer is right on the money for the codes:

P0171
P0174
P0300

I'll see what I can do about finding the problem now. Would this hurt my engine to keep running it while getting these intermittent codes?

[Injuneer]

Two possibilities....

It really is running lean - The long terms have max'd out at 160, and the short term fuel corrections are having to keep it from running lean. Maybe they can, maybe they can't. Running lean will increase the chances of knock (detonation). The knock sensor would probably protect your engine from damage. In cases of severe lean - enough to cause misfires - you run the risks of burning spark plugs and valves.

Its a "false" lean, caused by misifres, exhaust leaks or faulty O2 sensors - the PCM starts to add fuel the engine doesn't need. Wastes fuel, carbons up plugs, can damage the cats, and in severe cases can wash the lubrication of the cylinder walls and destroy the rings.

Not good to risk driving for too long. I had a "lean" exhaust code on my wife's Jeep Grand Cherokee.... but it was stumbling and pouring out unburned fuel. Put a scanner on it, confirmed the code, cleared the code and it started to run right. After a few minutes, as I watched the long term fuel corrections start to build. Checked the O2 sensor reading, and the voltage was "stuck" at around 280-300mV. Sensor was telling the PCM the exhaust was lean, but it wasn't - the sensor was stuck. Turned it off and told her not to drive it until I could get a new sensor. When I pulled the old sensor, the outside of it was coated with some power steering fluid that had been leaking. It was enough to block the air vent that the sensor needs to allow fresh air into the inside of the zirconium thimble. Popped in the new sensor and it ran fine.

Does the exhaust smell of fuel, even at idle? Does it pour out black smoke when you accelerate? Both those would indicate a "false lean", and I would stop driving it, or at least minimize driving it until you found the cause.

[birdOFprey]

i had the same codes running on mine for a while and it turned out to be the cats and O2 sensors...